Foward facing factory plenum mod

[Bo0sT_JuNkY]

Im gona check out if i could slice the bottom bit of the stock plenum then weld it back so its mirror'd on the other side.Only thing i think would be a problem is fitting the T/B and bending the charge pipe to clear the strut tower?Im gona look now.If i cant then im just gona run the fmic piping like stock with the extra pipe ontop of the fmic outside the car instead of being in the engine bay.Making a good flowing plenum with even airflow to each cylinder isnt to hard,It's try to make one that u can fit into the engine bay that is.

[Sydneykid]

Hi Roy, I have heaps of dyno charts, but response and airflow distribution don't show up on the rolling road. So they aren't of any use. I have tried lowering the ramp angle, decreasing the inertia loading and jumping on and off the throttle at various RPM's. You get some interesting graphs, but I haven't got to the stage where I can reliable measure throttle response on the dyno that compares with what it feels like on the circuit.

I’ll get to the other questions in a later post

[Sydneykid]

Hi Gradenko, excellent response, a good healthy debate, makes you want to get up in the morning...

Firstly I should point out that my comments refer to Nissan inline 6's and therefore may be applicable to other brands of straight 6 but not V6's or inline 4's. The distance from the front to rear of the engine is important here, so we shouldn't generalise and lump them all together and expect to get a reasonable result.

This is a Skyline forum so I won't dwell on it, but who says Toyota is right? They run the 1JZ's and 2JZ's so fat (full of fuel) that you would never get to the position of leaness in the rear cylinders. Plus if they were so good for response, why do they run the stupid sequential turbo stuff? To make up for the fact that the inlet manifold is poor for response? I'll leave to the individuals to draw their own conclusions, but "Toyota" and "response" don't sit very well with me. They specialise in "doughy" cars for the wally's.

I like the fact that you have kept the diameter of the pipework reasonable. You haven't fallen into the trap I see so often with others. They try and improve the response by shortening the pipe then increase the diameter so they loose all the benefit and then some. You obviously thought about what you were doing and what you were trying to achieve.

Nicely deflected question, sticking the BMW example in there. But you didn't answer the question.... How do you expect to get better throttle response from #6, #5, #4, when the relocated throttle body is further away from the inlet valves? There is more air between the TB and the inlet valve, therefore response must be worse.

Yes Fred was most definitely restricted by regulations. But Nissan saw fit to make what I consider to be the best circuit race exhaust manifold ever for the GTSR. The inlet manifold was also perceived to be a problem, which is why they went to the 6 port (instead of 12 port) design. But they didn't move the TB to the front, they could have, easily, but they didn't.

You are absolutely correct, the rear of an RB does have waterflow issues and the exhaust manifold and turbo orientation may well exacerbate this. So why add to it? Three wrongs don't make a right.

Sorry T0nyGTSt and rbs13, I will get to your questions in the next post.

[Sydneykid]

Hi rbs13, sorry for the delay, you asked the first questions....

It certainly helps the response, touch the throttle and you get instant results. At high power outputs (over 750 bhp in our experience) you start to get a couple of problems. The first one is the throttle bodies are starting to become restrictive ie; too small for the airflow. Secondly at wide open throttle (WOT) the previously very minor distribution problems of the standard plenum start to become noticeable, with #6 (and to a lessor extent #5) starting to receive excess air flow (leaning out).

Now 750 bhp is well oustide the requirements for good throttle response for a road and/or circuit race car , that was gone 100 bhp ago. This is not realy a problem for a drag car, they don't need throttle response, they should be at WOT for the 99.9% of the 1/4.

So what do they do? They generally remove the standard 6 X throttle bodies (I have seem a couple with larger butterflies, but that is unusual) and stick a > 80 mm one at the front of the standard plenum. The next step up is to replace the whole plenum with a larger aftermarket item (the Veilside plenum is the best design I have seen). Although I saw the Apexi GTR do an 8.75 with the standard plenum, so I am not even sure that you need to replace it. BTW that car used AFM's and a Power FC, it killed a few myths all at once.

Hope that answered your question.

[Sydneykid]

Hi T0nyGTSt, sorry I left your question until last...

I believe that keeping the distance form the throttle body to the inlet valve as short as possible is most important, as is keeping all cylinders the same. Keeping the distance from the turbo outlet to the throttle body is also very important. I don't believe the distance from the air filter to the turbo is anywhere near as important, as long as there is no restriction of course.

If you ever get the chance you should try running a pipe straight from the turbo to the throttle body. You won't believe the improvement in response. It will of course ping its brains out because of the huge increase in inlet air temperature, so you wouldn't want to do it for very long. Plus it looses a heap of horsepower, but the response has to be felt to be believed.

When we talk about response we are not talking about very much time here, at 5,000 rpm the engine is firing a cylinder every 1/40th of second. So an engine with poor response (and 10 litres of air in its inlet system) might take 1/10th of second longer to respond to throttle input than a highly responsive engine (with 8 litres of air in its inlet system). Depending on your feel and point in time requirements, 1/10th of a second can be nothing or an eternity.

Hope that helps

[Gradenko]

Originally posted by Roy

By shpowing dyno graphs im not expecting it to show any difference in power, but hopefull you will have some graphs with boost pressure shown on them, a means of telling if the thing is more tractable/ throttle responsive.

I do have a dyno graph with power and boost on it, but I'm not going to show it. Nothing more confusing than people trying to compare rwhp, so I'm making a stand against it. :)Besides, the graph tells us nothing about part throttle or low load situations.

I've already mentioned that it makes 13psi by 2700rpm, full throttle in 4th gear on the dyno. I don't have a "before" graph to compare it to.

[RNS11Z]

Would a twin throttle set up help??

[Gradenko]

Sydneykid, come on, 1jz and 2jz engines fat and unresponsive? The early 1jz's were simultaneous twin turbo'd, not the clumsy sequentional style of the 2jz. The 1jz's made 70% of their max torque by 3000rpm, and they sure didn't feel lethargic to drive. Sure, Toyota may have run them rich from factory, but can't the same be said of the RB25? (Yes!) Its the same story for most late model turbo cars, run them rich and save on warantee repairs. Actually, I remember accusing Nissan of hiding flaws with factory plenum by running the engine overly rich in another thread - you're borrowing my ideas Sydneykid

I don't mind restricting this conversation to Nissan inline 6's, but it is interesting to note that Nissan used the crossover plenum design on fwd ca18det's and then switched to a forward facing deign for rwd ca18det's. They had no problem changing the design for little else than packaging purposes.

With the issue of volume between valves and throttle body, I'm not entirely sure it's an important point. Heres a quote from Roy in a previous thread:

It would be wrong to think of the inlet air as your garden hose blasting thru the throttle body favouring cylinders 3 & 4. Closer to the truth is that the inlet air is slowed to a point where the inlet plenum is a reservoir of air waiting for the inlet valves to open.

Using common sense, cylinder 4, 5 & 6 only have to look to the end of their runner for a fresh breath of air. Add forced induction into the equation and the distance between throttle body and intake runner should become a non-issue. I really have no experience with valve to throttle body distance and its affect on throttle response. Any references to published work?

With the issue of tuning so that some cylinders run rich while others make power, is this really all that different from factory plenum? If, for arguments sake, the modified plenum is indeed affecting distribution, running clyinders rich and robbing me of horsepower, I would consider it a fair trade off for the driveability and power gained from cooler intake temps. Wideband O2 sensors in each runner and an ecu that can trim indvidual injectors would be nice (even with the factory plenum), but I'm just looking for a driveable street car, not something to extract every last 10th from.

As an aside, I'm a fan of the gts-r's exhaust manifold too. PM me if you know of any used ones or aftermarket replicas around.

[Warpspeed]

Just been catching up with this thread, some very interesting views expressed here.

I still feel that a plenum with a front mounted throttle body can be designed to give pretty reasonable air distribution, but if you try to push it to extremes well beyond that for which it was originally designed, it is going to create some problems.

By the same token, a stock centre fed plenum is going to be a lot more tolerant of extreme power upgrades as Sydneykid says.

But I could never imagine anyone with a GTR replacing the stock six throttle body induction system with an RB25 induction system because it was "better".

It all really comes down to designing for your intended purpose, and realising that however you finally go about it, there are going to be some compromises and disadvantages.

[JimX]

Until reading some posts in this thread I had never even thought that the exhaust could make the rear cylinders run hotter and thus leaner due to the dump pipe being right next to them. I'm going to get some thermal wrap this weekend for the dump and see if it makes any difference. The uneven air distribution could be even less than I thought.

[Roy]

...another thing that may be factored into our thinking when trying to get our heads around the plenum thing is the firing order of the inline 6 cylinder.

On the modded front facing plenum, dont forget that simply becuse cylinder no.6 is furthest from the throttle body it isnt the last to suck air thru its inlet valves.

When we talk about response we are not talking about very much time here, at 5,000 rpm the engine is firing a cylinder every 1/40th of second.

The systems isnt static and when you consider a part of the process in isolation there will always be other elements to consider as not being negligible.

So to think air is rocketing thru the plenum straight down the runner of cylinder no.6, is neglecting that the incoming air is being utilised by the inlet valves of other cylinders.

[jtech]

can a gtr plenum fit onto a gtst? would that be a better option?

[B-Man]

Originally posted by jtech

can a gtr plenum fit onto a gtst? would that be a better option?

Nope - not without substantial modification, as I understand.

[T0nyGTSt]

Originally posted by jtech

can a gtr plenum fit onto a gtst? would that be a better option?

You'll also need the GT-R head.

$$$

T.

[Warpspeed]

It can be made to fit with an adapter plate, but it is a still pretty difficult thing to do though. A complete GTR head is a much better solution, and quite possibly no more expensive if you have to pay for all the machining involved in making up an adapter.

But it can be done, and has been done.

[Sydneykid]

Hi guys, I had a whole load of additional thoughts to post, counter arguments, theories and practicle examples.......but in the end I think we have all pretty much covered the different points of view.

So I might just sum up from my perspective;

There is only one advantage from moving the TB to the front and that is reduced pipework length. This may lead to improved response, but it does nothing else eg; there is no horsepower advantage up to the limit of the standard internals.

Conversley I won't call them dissadvantages, let's say risks instead;

1. Poor distribution or airlfow leading to lean A/F ratios in the rear cylinders and detonation

2. Or rich A/F ratios in the front cylinders costing horsepower

In my humble opinion routing the pipework efficiently eg; 120 degree bend at the TB, can almost equal the advantage of moving the TB to the front but without the risks.

Hope that clarifies my position.

[Gradenko]

Originally posted by Warpspeed

I still feel that a plenum with a front mounted throttle body can be designed to give pretty reasonable air distribution, but if you try to push it to extremes well beyond that for which it was originally designed, it is going to create some problems.

Pretty much the same assumption I was working under, that it should be able to hold 450hp - like a stock plenum - before having problems. But then RNS11Z wondered in with his modified factory plenum putting out a lot more than 450hp... Its interesting that each runner in his modified plenum flowed more than a factory plenum. Any internal plenum mods, RNS11Z, or just a relocated throttle body?

JimX, its the exhaust manifold itself (head to turbo) that may be causing problems, not the post-turbo exhaust. But wrap your dump anyway, it will make a difference in other areas .

Finally, Sydneykid. I largely agree with your last post, with only this to add as an advantage for the modified plenum: no heatsoak from radiator or cam covers. You'd have to weigh up how much power you'd gain from colder air vs how much you'd lose from certain cylinders running too rich. I've made my choice .

[Sydneykid]

Hi Gradenko, "I largely agree with your last post, with only this to add as an advantage for the modified plenum: no heatsoak from radiator or cam covers."

If you run a 120 degree bend at the the TB and insulate the pipe, there is no heat soak. I have tested it.

[ed]

damn this thread is a good read

keep it up.

[rev210]

sydneykid,

do have a pic of the 120 degree bend mod anywhere?